The present invention relates to a continuous casting mould with oscilla- tion device, in particular for a continuous steel casting machine with a curved casting axis in a vertical casting plane.

Background of the invention A conventional casting mould for a continuous steel casting machine with a curved casting axis typically includes a copper mould tube, which forms a curved casting channel along said curved casting axis, a mould housing surrounding the mould tube and a water cooling system within the mould housing for vigorously cooling the mould tube.

It is known to axially oscillate such a continuous casting mould in order to avoid sticking of the solidified shell to the mould tube, which would indeed result in surface defects in the cast strand and can even produce a liquid steei breakout. Experience has shown that best casting results are obtained if the continuous casting mould is subjected to axial oscillations with a frequency between 100 and 600 cycles per minute and an amplitude of 5 to 10 mm.

Furthermore, the path of these axial oscillations should follow as closely as possible the curved casting axis. Oscillation components in a direction trans- verse to the casting axis not only result in a poor surface quality of the cast strand, but also in an excessive wear of the mould tube.

In most prior art continuous steel casting machines, the continuous cast- ing mould is supported on a so-called oscillating table located below the mould.

Striving to produce oscillations along a curved casting axis, designers of such oscillating tables have devised very complex and cumbersome lever mecha- nisms for oscillating the support table of the mould. However, most of these oscillating tables do not completely succeed in avoiding the generation of transverse oscillations of the mould tube. Furthermore, due to the high masses to be oscillated and to high frictional loss in the lever mechanism, these oscillation tables are generally inadequate for producing oscillations with

frequencies higher than 200 cycles per minute.

US patent 5,715,888 describes a solution for replacing oscillation tables with a more compact and more efficient oscillation device. This oscillation device comprises an external support casing in which the continuous casting mould is axially supported by means of an annular, double acting pneumatic or hydraulic cylinder. The latter consists of two c-axial sleeves, which surround the continuous casting mould. These sleeves are axially movable relative to each other and are guided in this movement by a guiding device arranged between them. The housing of the continuous casting mould defines a shoulder with which it rests on the inner sleeve, whereas the outer sleeve is supported in the external support casing. It has to be appreciated that this compact oscilla- tion device allows to impose on the continuous casting mould oscillations with an amplitude of up to 10mm and frequencies higher than 200 cycles per minute. A drawback of this oscillation device is that the manufacturing of a big diameter annular pneumatic or hydraulic cylinder is rather expensive, in particular if the annular cylinder has to produce oscillations along a curved oscillation path.

US patent 5,676,194 describes a solution for making oscillation tables su- perfluous by integrating the oscillation device in the continuous casting mould.

A double-armed oscillating lever, which is pivotably supported by the mould housing, supports with one arm the mould tube within the mould housing and is connected with the other arm to a linear cylinder located outside the mould housing. Sealing elements, as e. g. metal diaphragms, are connected between the stationary housing and the mould tube, so as to allow an axial oscillation of the mould tube by means of the oscillating lever, while ensuring the sealing of a sealed cooling chamber around the mould tube. It follows that the mass to be oscillated is substantially reduced, that higher oscillation frequencies can be achieved and that the power consumption for oscillating the mould tube is reduced.

US patent 4,483,385 describes another solution for making oscillation ta- bles superfluous by integrating the oscillation device in the continuous casting

mould. The curved mould tube is surrounded by a housing including a spray cooling system for the mould tube. The lower end of the mould tube freely traverses a bottom opening of the housing. The upper end of the mould tube is secured to a substantially horizontal top plate. The latter is guided in four vertical guide pins protruding from a top frame of the housing. Two vertical cylinders are arranged in the mould housing on opposite sides of the mould tube. When activated, these two vertical cylinders lift the guided top plate from the top frame. Gravity causes the top plate to fall back onto the top frame, when the cylinders are deactivated. By activating and deactivating the two cylinders it is thus possible to subject the freely suspended mould tube to an oscillating up and down movement within the housing containing the spray cooling system. A serious disadvantage of the continuous casting mould described in US patent 4,483,385 is that the vertically oscillated curved mould tube exerts important lateral forces on the curved strand. These lateral forces result in excessive wear of the mould tube and surface deformations of the strand. They may even produce a break through of liquid metal.

Object of the invention A technical problem underlying the present invention is to provide a com- pact continuous casting mould that is capable of producing oscillations along a curved casting axis without exerting significant lateral forces on a strand leaving the curved mould. This problem is solved by a continuous casting mould as claimed in claim 1.

Summary of the invention The casting mould of the present invention comprises, in a manner known per se, a mould tube, which forms a curved casting channel along a curved casting axis of a casting machine, and a mould housing, which surrounds the mould tube and houses a cooling system for vigorously cooling the mould tube.

The mould tube has a support flange at is upper end. A pneumatic or hydraulic actuator means is connected between the support flange of the mould tube and

the housing for axially supporting and oscillating the mould tube in its stationary housing. In accordance with an important aspect of the present invention, the continuous casting mould further comprises a guiding device that is connected directly between the lower end of the mould tube and the bottom end of the mould housing. This guiding device imposes a curved oscillation path on the lower end of the mould tube (i. e. it fixes, at least in the casting plane, all translational and rotational degrees of freedom of the lower end of the mould tube), wherein the imposed oscillation path follows exactly the curved casting axis. In other words, the guiding device warrants that the oscillations of the mould tube, which are imposed on the upper end of the mould tube, result in oscillations of the lower end of the mould tube that strictly follow the curved casting axis. Consequently, the mould tube exerts no or at least no significant lateral forces on the curved strand leaving the mould. Within this context it remains to be noted that the oscillations imposed on the upper end of the mould tube must not necessarily follow exactly the path imposed on the lower end of the mould tube. Indeed, as the mould tube, which is normally made of copper, is not a very rigid body, it can easily compensate differences in the imposed oscillation paths of its lower and upper ends by small deformations transversal to the casting axis. It will be appreciated that these small transversal deformations do not deteriorate casting quality and do not result in an in- creased wear of the mould tube, because they mainly affect the upper mould tube where the steel is still entirely liquid.

The guiding device is preferably forcedly cooled and comprises, in a pre- ferred embodiment, at least one guiding element, which is fixed to the housing and includes a curved guiding channel, and a guided element, which is rigidly secured to the mould tube and guided in the curved guiding channel of the at least one guiding element. The guiding channel fixes all translational and rotational degrees of freedom of the guided element, so that the oscillations of the lower end of the mould tube strictly follow the curved casting axis.

The pneumatic or hydraulic actuator means is advantageously an annular, i. e. ring-shaped, pneumatic or hydraulic actuator cylinder, which is preferably forcedly cooled, so that it can directly surround the hot support flange of the

mould tube. It will therefore be appreciated that the annular cylinder can have a much smaller diameter than the annular cylinder disclosed in US patent 5,715,888, which is integrated in a support casing surrounding the whole housing of the mould. It will further be appreciated that the annular cylinder must not be able to produce oscillations along a curved path in the mould of the present invention. In other words, it can simply produce small strokes in a direction parallel to a rectilinear central axis. If this rectilinear central axis is substantially tangential to the curved oscillation path, the annular cylinder can-along its relatively small oscillation path-transmit substantially axial oscillation forces to the mould tube.

The upper end of the mould tube can be secured with functional play to the annular cylinder, so that oscillatory deformations of the tube are reduced or even completely avoided. It is however generally preferred to rigidly secure the mould tube to the annular cylinder in order to avoid any uncontrolled axial movement of the mould tube.

In order to reduce deformations of the mould tube at its upper end, the annular cylinder is advantageously supported on the top end of the housing so as to be capable of oscillating about an axis of rotation that is substantially perpendicular to the vertical casting plane and intersects the curved oscillation path. In a preferred embodiment, the annular cylinder e. g. has two journals and the top end of the housing has two bearings for receiving the journals. To reduce oscillatory deformations of the upper end of the mould tube, the annular cylinder may also be fixed to the top end of the housing, so as'to allow small linear displacements parallel to the vertical casting plane and transversal to the casting axis.

The cooling system of the mould is advantageously a spray cooling sys- tem. In a preferred embodiment of such a spray cooled continuous casting mould, a ring element is fixed to the lower end of the mould tube and arranged in a central cut-out of a bottom plate of the housing, so that a radial gap subsists between the ring and the bottom plate. As the lower end of the tube is guided by the guiding device, this radial gap remains substantially constant and

may therefore be sealed with an adequate sealing element.

Brief description of the drawings The present invention will now be described, by way of example, with ref- erence to the accompanying drawings, in which: Fig. 1: is a first schematic longitudinal section of a continuous casting mould with a curved casting axis, wherein the vertical section plane in- cludes the curved casting axis and is identified by section line A-A in Fig. 3; Fig. 2: is a second schematic longitudinal section of the continuous casting mould of Fig. 1, wherein the vertical section plane is perpendicular to the section plane of Fig. 1 and is identified by section line B-B in Fig. 3; and Fig. 3: is a schematic cross section of the continuous casting mould of Fig.

1 & 2, wherein the section line is identified with letters C-C in Fig. 1 & 2.

Detailed description of a preferred embodiment The Figures show a continuous casting mould 10 for casting steel billets in a continuous casting machine. This continuous casting machine defines a curved casting axis 11 in a vertical plane. It will be noted that the curved casting axis 11 physically corresponds to a substantially circular curve defined by the neutral axis of a bent strand in the casting machine.

Reference number 12 globally identifies a curved mould tube for receiving molten steel from a tundish (not shown), i. e. a refractory-lined liquid steel distributor, which is placed over the mould 10. This mould tube 12 comprises a copper tube 14 fixed with its upper end to a support flange 16, which forms a kind of inlet funnel 18. As shown on Fig. 1, which is a section along the vertical casting plane containing the casting axis 11, the copper tube 14 forms a casting channel 20 along the curved casting axis 11. In most continuous casting

machines, the curved casting axis can be equated in this region to a circular segment with a radius between 4 m and 12 m.

Reference number 24 globally identifies a cylindrical mould housing sur- rounding the curved copper tube 14. This mould housing 24 houses a known spray cooling system 26 for vigorously cooling the copper tube 14. This spray cooling system 26 comprises a set of vertical cooling water pipes 28 extending from an annular collector at the bottom end of the housing 24 to its top end.

Each of these pipes includes a series of spray nozzles 29, which serve to spray the copper tube 14 with cooling water.

Reference number 30 globally identifies an annular, i. e. ring-shaped, hy- draulic cylinder capable of producing linear strokes. This annular cylinder comprises an outer ring 32 and an inner ring 34. By means of its outer ring 32, the cylinder 30 is supported on the top end of the housing 24. The mould tube 12 is supported by means of its support flange 16 on a flange 36 of the inner ring 34. The rings 32,34 are capable of a rectilinear movement relative to each other in a direction parallel to a central rectilinear cylinder axis 35. They co- operate to form an annular piston 38 axially separating two annular pressure chambers 40,42, thus forming a double-acting hydraulic cylinder capable of producing axial forces in two opposite directions (see arrows 44) along the central cylinder axis 35. It will further be noted that the central cylinder axis 35 of the annular cylinder 30 is substantially tangential to the curved casting axis 11 at the point"P"where a plane of symmetry 45 of the two annular piston chambers 40,42 intersects the curved casting axis 11. It follows that the annular cylinder 30 can-along its relatively small oscillation path-transmit substantially axial oscillation forces to the mould tube 12.

As best seen on Fig. 2, the outer ring 32 of the cylinder 30 is supported on the top end of the housing 24 by means of two journals 46,48. Each of these journals is received in a bearing 50,52, which is fixed to the housing 24. The journals 46,48 and the bearings 50,52 are arranged so as to define an axis of rotation 54 for the annular cylinder 30, which is substantially perpendicular to the vertical casting plane and intersects the casting axis at the point"P"defined

above. The outer ring 32 defines an annular gap with a ring-shaped flange 56 of the housing 24, which is closed by an elastically deformable seal ring 58. It follows that the annular cylinder 30 is capable changing its position by rotating a small angle about its axis of rotation 54, while the elastically deformable seal ring 58 continuously seals the gap between the outer ring 32 of the cylinder 30 and the ring-shaped flange 56 of the housing 24.

The inner ring 34 surrounds the support flange 16 of the mould tube 12, wherein there remains an annular gap between the support flange 16 and the inner ring 34. This annular gap is at least partially filled with a refractory lining 60, which protects the annular cylinder 30 against radiant heat from the support flange 16. Heat protection of the annular cylinder 30 is further improved by equipping both rings 32,34 with an internal cooling circuit (not shown) and/or by providing a series of spray nozzles 62 in the housing 24 for spraying a cooling fluid onto the underside of the annular cylinder 30.

In Fig. 2 and Fig. 3, reference number 63 globally identifies a guiding de- vice that is connected directly between the lower end of the copper tube 14 and the bottom end of the mould housing 24. The object of this guiding device 63 is to impose a curved oscillation path on the lower end of the copper tube 14, wherein this imposed oscillation path follows exactly the curved casting axis 11 in this region. In order to achieve this, the guiding device 63 e. g. comprises two U-shaped guiding elements 64,66, which are fixed to a bottom plate 67 of the mould housing 24, symmetrically with regard to the vertical casting plane. Each of these U-shaped guiding elements 64,66 includes a curved guiding channel; i. e. a channel delimited by two cylindrical guiding surfaces 64', 64", respectively 66', 66", and a plane base surface 64"', 66"'. It will be noted that the two cylindrical guiding surfaces 64', 64", respectively 66', 66", have their axis of revolution perpendicular to the casting plane (plane A-A) and passing through the centre of the circular segment that represents the casting axis 11 in this region. In each of the two guiding channels is received a guided element 68,70 that is rigidly secured to the mould tube 12 and guided in the curved guiding channel of the respective guiding element 64,66 by the cylindrical guiding surfaces 64', 64", respectively 66', 66" (i. e. the cylindrical guiding surfaces 64',

64", 66', 66"fix all translational and rotational degrees of freedom of the lower end of the mould tube). In other words, the curved lateral guiding surfaces 64', 64", respectively 66', 66", are designed so as to impose, via the guided elements 68,70, an oscillation path with a curvature corresponding essentially to the curvature of the curved casting axis 11 on the lower end of the copper tube 14. This means that the oscillation path of the lower end of the copper tube 14 exactly follows the curved casting axis 11.

As best shown in Fig. 3, the guided elements 68,70 are supported by a ring element 72 that surrounds the lower end of the copper tube 12. As best shown in Fig. 1 and Fig. 2, this ring element 72 is fixed to the copper tube 12 by means of gibs 74 engaging grooves in the wall of the copper tube 14. It will be noted that the guiding elements 64,66 and the guided elements 68,70 are designed so that a continuous flow of cooling water streaming out of the bottom end of the mould housing 24 cools these elements 64,66,68,70. Schematic spray nozzles 76 indicate that the guiding device 63 is also subjected to a forced cooling from the underside.

Referring now to Fig. 1 and Fig. 3, it will further be noted that the ring ele- ment 72 is arranged in a central cut-out of the bottom plate 67, wherein a radial gap, which subsists between the ring 72 and the bottom plate 67, is sealed by a sealing element, preferably a graphite seal 78.

When the two annular pressure chambers 40,42 are alternately pressur- ised and depressurised, the annular cylinder 30 imposes an oscillating move- ment on the mould tube 12. At the height of the lower end of the copper tube 14, the guiding device 63 warrants that the oscillations, which are imposed on the upper end of the mould tube 12, result in oscillations of the lower end of the copper tube 14 that strictly follow the curved casting axis 11. In other words, the guiding device 63 warrants that the copper tube 14 does not exert significant lateral forces on the curved strand leaving the mould tube 12. At the upper end of the mould tube 12, the oscillation path is, at least in the device described above, not completely identical to the casting axis 11. This difference in the oscillation paths of the lower and upper end of the mould tube 12 becomes

possible because the copper tube 14 is not a very rigid body and can therefore be easily subjected to small oscillatory deformations, which compensate differences in the oscillation paths. These oscillatory deformations, which mainly take place in the upper mould tube 12, do not significantly affect the quality of the cast product, because the liquid steel level, indicated by arrow 79 in Fig. 1 & 2, is located roughly 150 mm below the axis of rotation 54. There- fore, it is sufficient to warrant that the stability of the copper tube 14 is not affected by these oscillatory deformations. This is the reason why the annular cylinder 30 should advantageously have an axis of rotation 54 as described above. It can indeed be shown that a possibility of rotation about the axis 54 helps to reduce the deformations of the copper tube 14 by about 50 %. If a further reduction of the deformations of the copper tube 14 is required, then it is e. g. possible to fix the annular cylinder 30 to the top end of the housing 24, respectively the support flange 16 of the mould 12 to the flange 36 of the inner ring 34, so as to allow small linear displacements (size range generally less than 1 mm) parallel to the vertical casting plane and transversal to the casting axis 11 (see Fig. 1, arrow 80).

A continuous casting mould 10 of the type described above has mean- while been successfully tested. It is capable of producing oscillations with an amplitude from 1-20 mm and frequencies up to 600 cycles per minute. It distinguishes itself not only by a very high movement accuracy, but also by a low lubricant consumption, a very compact and simple layout, a quick copper tube exchange, a quick exchange of the oscillation device in case of a break- down of the latter, and low maintenance costs, because each component may be very easily exchanged.